These studies are aimed at assessing the role of protein-protein interactions in the structure of Escherichia coli ribosomes. Solutions of ribosomal proteins are examined in the absence of ribosomal RNA. These solutions are exposed to reversible cross-linking reagents, and cross-linked complexes are detected by diagonal gel electrophoresis. Also, various ribosomal proteins which have been implicated in protein-protein interactions are being purified. Mixtures of these proteins will be examined by sedimentation equilibrium in the analytical ultracentrifuge and molecular weight determinations will be used to detect complex formation. Other studies are aimed at exploring the utility of certain bifunctional reagents in covalently cross-linking ribosomal proteins to ribosomal RNA, to aid in locating the RNA binding sites of various proteins.